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Chapter 2

PSYB64 - Chapter 2

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Pare, Dwayne

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PSYB64 – Chapter 2: The Anatomy and Evolution of the Nervous System Anatomical Directions and Planes of Section  The following are for 4 legged animals: the neuraxis forms a straight line running parallel to the ground  rostral or anterior = structures that are located toward the head end of the animal o ex. Head of the dog is rostral to its shoulders  caudal or posterior = structures located toward the tail end of the animal o ex. The dog’s ears are caudal to its nose and its hips are caudal to its shoulders  inferior or ventral = structures located toward the belly side  superior of dorsal = structures towards the back  the dorsal parts of the animal’s brain are in line with the dorsal parts of the spinal cord  for people, the directions are different because our 2 legged stance puts a 90 degree bend in the neuraxis (an imaginary line that runs the length of the spinal cord through the brain)  in humans, the dorsal parts of our brain form a 90 degree angle with the dorsal parts of the spinal cord  midline = an imaginary line that divides us into approximately equal haves  ipsilateral = structures are both on the same side of the midline ex. Left arm and left leg are ipsilateral  contralateral = structures are on opposite sides of the midline ex. Left arm and right leg are contralateral  medial = structures close to the midline ex. Heart is medial to arms  lateral = structures to the side of the midline ex. Ears are lateral to nose  proximal = close to the center ex. Shoulders are proximal relative to elbows  distal = far from the center ex. Toes are distal relative to knees  there are 3 major planes of section o coronal = frontal sections; divide nervous system from front to back (ex. Looking at brain in the face) o sagittal = parallel to the midline; allow us to see a side view of the brain structures o horizontal or axial = divides the brain from top to bottom Protecting and Supplying  the bony skull protects from all but the most serious blows  the skull bones are not fully mature in infants; they are born with skull bones that can overlap each other, somewhat like the tectonic plates of the Earth o this aids the movement of the baby’s head through the birth canal o takes about 18 months for the human skull to fuse completely Meninges  these are layers of membranes that surround the nervous system and aid in protection  the 3 layers are… o dura mater = “hard mother”; outermost layer; composed of leather like tissue that follows the outlines of the skull bones o arachnoid layer = below the dura mater; delicate layer that looks like a spider’s web o pia mater = “pious mother”; nearly transparent membrane that sticks closely to outside of brain  between arachnoid and pia mater layers is the subarachnoid space  all 3 layers cover the brain and spinal cord BUT ONLY DURA MATER AND PIA MATER COVER NERVES THAT EXIT THE BRAIN (peripheral nervous system)  meningitis = condition in which the meninges get infected o it could be life-threatening  when tumors arise in the tissue of the meninges, they are referred to as meningiomas Cerebrospinal Fluid (CSF)  CSF is secreted within hollow spaces in the brain called ventricles  Within the lining of the ventricles, the choroid plexus coverts material from the nearby blood supply into CSF  CSF is similar in composition to blood plasma and because of its weight and composition, it essential floats in the brain within the skull. This has advantages… o If you bump your head, the fluid acts like a cushion to soften the blow o Neurons respond to appropriate input, not to pressure on the brain. Pressure can often cause neurons to fire in maladaptive ways such as when a tumor causes seizures by pressing down on a part of the brain o The CSF prevents neurons from responding to pressure and providing false info  CSF circulates through the central canal of the spinal cord and the 4 ventricles in the brain (two lateral, one in each hemisphere, and 3 and 4 in the brain stem)  The 4 ventricle is continuous with the central canal of the spinal cord which runs the length of the cord atthts midline  Below the 4 ventricle there is a small opening that allows the CSF to flow into the subarachnoid space that surrounds both the brain and the spinal cord  New CSF is made constantly, with the entire supply being turned over about 3 times per day and the old CSF is reabsorbed into the blood supply at the top of the head  Blockages can occur in the narrow sections of the circulation system of CSF and this is known and hydrocephalus  Hydrocephalus means water on the brain  Left untreated, it can cause mental retardation as the large amount of CSF prevents the normal growth of the brain  It can be treated by installing a shunt to drain off excess fluid and surgery can be used to repair the obstruction in the baby when it is older  CSF is a separate circulation system that NEVER HAS direct contact with the blood supply  In a spinal tap procedure, the physician withdraws some fluid from the subarachnoid space through a needle because the composition of CSF is important for diagnosing some diseases Blood Supply  Brain receives its nutrients from the carotid arteries on either side of the neck and through the vertebral arteries that travel up through the back of the skull  once inside the skull, these major arteries brain to form the anterior, middle and posterior cerebral arteries which serve most the brain  any interruption of blood supply produces damage very quickly because the brain doesn’t store energy  Significant brain damage happens less than 3 mins. after the stopping of the heart whereas other structures of the body are not affected as quickly  other organs are able to continue indefinitely with life support that’s why brain death is a working definition of death The Central Nervous System  the entire nervous system is divided into 2 components, CNS and PNS  CNS = brain + spinal cord; tissue is encased in bone  PNS = nerves that exit the brain and spinal cord, carrying sensory and motor messages to and from the other parts of the body; tissue is NOT encased in bone The Spinal Cord  Long cylinder of nerve tissue that extends from the medulla down to the first lumbar vertebra  Neurons that make up the spinal cord are found in the upper 2/3rds of the vertebral column  The spinal cord is shorter than the vertebral column because the cord itself stops growing before the bones in the vertebral column do  Running down the center of the spinal cord is the central canal  The spinal nerves exit between the bones of the vertebral column and the bones are cushioned from one another with disks o If any of the disks degenerate, pressure is exerted on the adjacent spinal nerves, producing a pinched nerve  Spinal cord is divided into 31 segments o 8 cervical nerves that serve the area of the head, neck, and arms o 12 thoracic nerves serve the torso o 5lumbar nerves serve the lower back and legs o 5 sacral nerves serve the backs of the legs and genitals o 1 coccygeal nerve  The spinal cord is the original information superhighway  Much of the spinal cord appears white o White matter is made up of nerve fibers known as axons o The tissue looks white due to fatty material known as myelin o When the tissue is preserved for study, the myelin repels staining and remains white, looking much like the fat on steak o The large bundles of axons are responsible for carrying info about touch, position, pain and temperature travel up the dorsal parts of the spinal cord o Axons of motor neurons are responsible for movement, travel in the ventral parts of the cord  The center of the cord looks like a gray butterfly or letter H o Gray matter consists of area primarily made up of cell bodies o The tissue appears gray because the cell bodies absorb some of the chemicals used to preserve the tissue, which stains them gray o The neurons found in the dorsal horns of the H receive sensory input, whereas neurons in the ventral horns of the H pass motor info on to the muscle o These ventral horn cells participate in either voluntary movement or spinal reflex  Without any input from the brain the spinal cord neurons are capable of some important reflexes o Patellar reflex = knee jerk; this reflex is managed by only 2 neurons, one neuron processes sensory info coming to the cord from muscle stretch receptors (this neuron communicates with a spinal motor neuron that responds to input by contracting a muscle, causing the foot to kick) o Withdrawal reflex = protects from injury; you immediately pill your body away from the source of the pain. Three neurons are involved, sensory, motor, and interneuron o Because so few neurons are involved, the effects produce very rapid movement o The spinal cord manages a number of more complex postural reflexes that help us stand and walk o They also allow us to shift our weight automatically from one leg to another when walking  Damage to the spinal cord results in los of sensation and loss of voluntary movement in parts of the body served by nerves located below the damaged area  A person with cervical damage is a quadriplegic o All sensation and ability to move the arms, legs and torso are lost  A person with lumbar level damage is a paraplegic o Use of the arms and torso is maintained but sensation and movement in the lower torso and legs are lost  In all cases of spinal injury, bladder and bowel functions are no longer under voluntary control as input from the brain to the sphincter muscles does not occur  Spinal damage is considered permanent The Hindbrain  Early in embryological development, the brain divides into 3 parts: the hindbrain, midbrain (mesencephalon) and forebrain  Hindbrain + midbrain = brainstem  Later in embryological development, the midbrain makes no further divisions  Hindbrain = myelencephalon, or medulla + metencephalon  The Myelencephalon (Medulla) o Contains large quantities of white matter o Instead of the butterfly appearance of the gray matter in the spinal cord, the medullar contains a number of nuclei which are suspended within the white matter of the medulla o Some these nuclei contain cell bodies whose axons make up several of the cranial nerves serving the head and neck area o Along the midline of the upper medulla, there is the reticular formation which is a complex collection of nuclei that runs along the midline of the brainstem from the medulla up into the midbrain and plays an important role in the regulation of sleep and arousal  The Metencephalon o Contains the pons and the cerebellum o Pons lie immediately rostral to the medulla and they form connections between the medulla and higher brain centers as well as with the cerebellum o Large fiber pathways with embedded nuclei are found in the pons  Cochlear nucleus = receives info about sound  vestibular nucleus = receives info about position and movement of head and helps keep our balance o reticular formation extends through the pons and on into the midbrain o nuclei located within the pons are necessary for the production of REM sleep o raphe nucleus and locus coeruleus project widely to the rest of the brain and influence mood, states of arousal and sleep o the cerebellum looks like a second little brain attached to the dorsal surface of the brainstem (the internal structure resembles a tree) o cerebellum has a role in coordinating voluntary movements, maintaining muscle tone, and regulating balance o input from the spinal cord tells the cerebellum about the current location of the body in a 3D space o input from the cerebral cortex by the way of pons tells the cerebellum about movements you intend to make o the cerebellum then processes the sequences and timing of muscle movements required to carry out the plan o damage of the cerebellum affects skilled movements, including speech production and because the cerebellum is one of the first structures to be affected by the consumption of alcohol, most sobriety tests are actually tests of cerebral function o cerebellum also contributes to the experience of motion sickness o cerebellum is also involved in some of the more sophisticated processing of info o one of the embedded nuclei of the cerebellum (dentate nucleus) has become particularly large in monkeys and humans and a part of the dentate nucleus (neodentate) is found only in humans o patients with cerebral damage also experience subtle deficits in cognition and perception o in cases of autism, a reliable anatomical marker is an abnormal cerebellum The Midbrain (mesencephalon)  has a dorsal or top half (tectum) and a ventral or bottom half (tegmentum)  In the midbrain, CSF is contained in a small channel at the midline known as the cerebral aqueduct. The cerebral aqueduct separates the tectum from the tegmentum and links the 3 th and 4 ventricles  Surrounding the aqueduct are cell bodies known as periaqueductal gray and it appears to play an important role in our perception of pain  There are large numbers of receptors in the periaqueductal gray that respond to opiates  Electrical stimulation of this area provides considerable relief from pain  The midbrain contains the most rostral portion of the reticular formation and a number of nuclei associated with cranial nerves  Red nucleus and substantia nigra are nuclei found in the midbrain o Red nucleus (located within the reticular formation) communicates motor info between the spinal cord and the cerebellum o Substantia nigra is closely connected with the basal ganglia of the forebrain and this is what degenerates in Parkinson’s disease (causes difficulty moving)  There are 4 prominent bumps on the surface of the midbrain o Superior colliculi – receive input from the optic nerves leaving the eye; they allow us to make visualy guided movements such as pointing in a direction; they also participate in visual reflexes o Inferior colliculi – involved with hearing; involved with auditory reflexes such as turning head in the direction of sound; appear to participate in localization of sounds in the environment by comparing the timing of arrival of sounds at the two ears The Forebrain  Contains the most advanced and most recently evolved structures of the brain  It divides in embryological development o Diencephalon – thalamus + hypothalamus o Telencephalon – contains the bulk of symmetrical left and right cerebral hemispheres  The Thalamus and Hypothalamus o Diencephalon is located at the rostral end of the brainstem and the upper portion consists of the thalamus o There are 2 thalamic nuclei o Inputs from most of our sensory systems converge on the thalamus which then forwards the info on to the cerebral cortex for further processing o The thalamus doesn’t change the info, it just filters the info passed along to the cortex depending on the organism’s state of arousal o The cerebral cortex forms large numbers of connections with the thalamus o The thalamus is also involved with states of arousal and consciousness o Damage to the thalamus results in a coma and some seizures o Thalamus is also involved in learning and memory o Below the thalamus is the hypothalamus which is a major regulatory center for behaviors such as eating, drinking, sex, biorhythms and temp. control. o The hypothalamus is a collection of nuclei and it is directly connected to the pituitary gland from which many important hormones are released o The hypothalamus also directs the autonomic nervous system  The Basal Ganglia o Made up of several nuclei which include caudate nucleus, putamen, globus pallidus and subthalamic nucleus o Sometimes the substantia nigra are included in this too because it is closely connected with these nuclei o Involved in motor control o The nucleus accumbens also plays a role in experience of reward o Basal ganglia are important part of the motor system and degene
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